Automotive vehicles with an internal combustion engine have an exhaust system including a pathway for exhaust gas to move away from the engine. Depending on the desired operating state, internal combustion engines can be operated with fuel/air ratios in which (1) the fuel constituent is present in a stoichiometric surplus (rich range), (2) the oxygen of the air constituent is stoichiometrically predominant (lean range), and (3) the fuel and air constituents satisfy stoichiometric requirements. The composition of the fuel-air mixture determines the composition of the exhaust gas.
The oxygen concentration in the exhaust gas of an engine has a direct relationship to the air-to-fuel ratio of the fuel mixture supplied to the engine. As a result, gas sensors, namely oxygen sensors, are used in automotive internal combustion control systems to provide accurate oxygen concentration measurements of automobile exhaust gases for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
An oxygen sensor comprises an ionically conductive solid electrolyte material, a sensing electrode that is exposed to the exhaust gas and reference electrode that is exposed to a reference gas, such as air or oxygen, at known partial pressure. It operates in potentiometric mode, where oxygen partial pressure differences between the exhaust gas and reference gas on opposing faces of the electrochemical cell develop an electromotive force (EMF), which can be described by the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2              ref                                            P                          O              2                                      )            
where:                               E          =                      electromotive force                                                        R          =                      universal gas constant                                                        F          =                      Faraday constant                                                        T          =                      absolute temperature of the gas                                                                    P                          O              2                        ref                    =                      oxygen partial pressure of the reference gas                                                                    P                          O              2                                =                      oxygen partial pressure of the exhaust gas                                "AutoLeftMatch"
The large oxygen partial pressure difference between rich and lean exhaust gas conditions creates a step-like difference in cell output at the stoichiometric point.
Oxygen sensors, during operations, are subjected to varying conditions such as temperatures ranging from ambient temperatures, when the engine has not been recently run, to higher than 1,000xc2x0 C. during operation. Certain properties of the sensor may be affected by the varying conditions including electrical parameters, namely voltage amplitude, response times, switching characteristics, and light-off times. As such, stable and reproducible performance of a sensor is desirable.
Disclosed herein is a method for producing a gas sensor, comprising disposing a reference electrode on a side of an electrolyte, disposing a measuring electrode on a side of the electrolyte opposite the reference electrode, disposing a first protective coating on a side of the measuring electrode opposite the electrolyte, treating the sensor with an aqueous salt solution comprising chloride and carbonate salts comprising elements selected from the group consisting of Group IA and IIA elements of the Periodic Table to form a treated sensor comprising the chloride and the carbonate salt mixture, drying the treated sensor, and disposing a second protective coating on a side of the first protective coating opposite the measuring electrode.